Archive image from page 589 of The cyclopædia of anatomy and. The cyclopædia of anatomy and physiology cyclopdiaofana0401todd Year: 1847 568 SHELL. structure in the calcareous rosette, with which, as long since observed by Monro, the sucker at the extremity of each am- bulacral tube is furnished. But it is in the spines with which the shell is beset, that the most remarkable displays of it are to be met with ; for it is there disposed in connection with solid ribs or pillars, which increase the strength of these organs, in such a manner as to constitute a most regular and elaborate pattern, w
Archive image from page 589 of The cyclopædia of anatomy and. The cyclopædia of anatomy and physiology cyclopdiaofana0401todd Year: 1847 568 SHELL. structure in the calcareous rosette, with which, as long since observed by Monro, the sucker at the extremity of each am- bulacral tube is furnished. But it is in the spines with which the shell is beset, that the most remarkable displays of it are to be met with ; for it is there disposed in connection with solid ribs or pillars, which increase the strength of these organs, in such a manner as to constitute a most regular and elaborate pattern, which appears to differ in every distinct species. When we make a thin transverse section of almost any spine be- longing to the genus Echinus, we are at once made aware of the existence of a number of concentric layers, arranged in a manner that strongly reminds us of the layers of wood in the stem of an exogenous tree. The number of these layers is extremely variable ; depending, not merely upon the age of the spine, but upon the part of its length from which the section is taken. The centre of the spine {fig. 424. a.) is filled up Fie. 424. Transverse section of spine of Echinus : a, medullary centre ; bb, first layer of solid pillars; cc, dd, ee, ff, successive rings of growth. Magnified 45 diameters. with the same kind of calcareous net-work as that of which the shell is composed ; and this is sometimes so delicate, as to appear as if made up by the interlacement of mere threads. This medullary centre is bounded by a row, more or less circular according to the form of the spine (which is sometimes angular), of open spots (b, b, b), in which it is deficient : these, on a cursory inspection, might be supposed, from their transparency, to be void spaces ; but a closer inspection makes it evident that they are the sections of a circular row of solid ribs or pillars, which form the exterior of every layer. Their solidity becomes very obvious when we either examine a section
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